Mining carts and excavation equipment at the tunneling area of the North's Punggye-ri site can be seen in satellite images taken by Digital Globe and GeoEye in the past month. Earth and debris are being removed from the tunnel in the largest quantities seen so far, according to the Janes assessment.

"Based on the sand piles from the commercial satellites, we are assuming that they have put necessary devices for a nuclear test inside the shaft," Kim Min-seok, spokesperson for South Korea's defense ministry said in a regular briefing Thursday. "This means they can conduct a nuclear test any time."

He said North Korea was "awaiting a political decision" on whether or not to go ahead with the test, the prospect of which has strained relations between Pyongyang and other countries.

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Many analysts assume an atomic test by North Korea is just a matter of time following the failure of a controversial rocket launch last month. Two previous rocket launches in 2006 and 2009 were followed weeks or months later by nuclear tests.

A satellite image from mid-April cited by Janes shows a full mining train, including an engine and several carts, outside of the tunnel at the nuclear site. And a more recent shot on May 9 reveals new road networks at the site along with carts and a vehicle at the facility.

An unidentified North Korean Foreign Ministry official suggested that a nuclear test had not originally been part of the regime's plans.

"We did not envisage such a military measure as a nuclear test as we planned to launch a scientific and technical satellite for peaceful purposes," he said in a report Tuesday by the state-run Korean Central News Agency.

But the statement concluded with a vague threat: "If the U.S. persists in its moves to ratchet up sanctions and pressure upon us despite our peace-loving efforts, we will be left with no option but to take counter-measures for self-defense."

North Korea launched a rocket on April 13, which failed less than two minutes into the flight. It said the launch was to put a satellite into orbit, but much of the international community saw it as a cover up for testing its ballistic missile technology.

The move torpedoed a deal reached in February under which Pyongyang agreed to suspend its nuclear activities in exchange for food aid shipments from the United States.

The rocket launch in April was timed to coincide with the 100th anniversary of the birth of Kim Il Sung, the founder and longtime leader of North Korea.

It also came a few months after the death in December of Kim Jong Il, who had headed the secretive regime since 1994. His son and chosen successor, Kim Jong Un, has taken over as "supreme leader" of the nation, but the level of his influence on policy decisions remains unclear.